virgl: Add tweak to disable use of coherent buffer access

Allocating buffers with coherent memory access require blob resources, and these use file descriptors to share the resource between host and guest. If many small buffers are allocated we exhaust the number of available file descriptors, but if an application doesn't have a hard requirement on coherent buffer access, and the access pattern of the buffers allows it we can avoid this problem by disabling coherent buffer access.

Enable the tweak for "Total War: Warhammer"